This invention relates to a numerical control machining method and apparatus wherein a workpiece is machined on the basis of curved surface data defining a curved surface. More particularly, it relates to a numerical control machining method and apparatus wherein an appointed boundary surface is entered as an upper limit or a lower limit for the machining. A determination is made whether a tool lies above or below the boundary surface to machine an upper or lower curved surface with respect to the boundary surface on the basis of the curved surface data which specifies the curved surface when the tool lies above or below the boundary surface.
When machining a metal mold in a three-dimensional shape by means of a profiling machine, it is generally impossible to immediately start the finish machining from a solid material, because of the cutting performance of a machine tool. It is therefore a general practice to rough machine the material in advance, or to finish the mold in several stages by cutting the material in small increments.
A clamp profiling system has been previously proposed in order to perform the rough machining and the finish machining or to perform the several machining stages, as described above, and it has demonstrated its effectiveness. The clamp profiling system is a method wherein a clamp height is previously set so that the material may be cut down to a depth to which the machine tool can cut, the clamp height is lowered at each profiling operation, and the machining of the whole surface of the three-dimensional shape is completed by several profiling operations. With this system, however, the tool strokes the already cut surfaces at low speed in the second and subsequent operations and executes useless motion, which leads to the disadvantage of prolonged machining time.
On the other hand, in a situation where NC (numerical control) command data for machining the curved surface of a metal mold is automatically created by the use of an automatic programming system and where the metal mold is machined on the basis of the NC command data, a first part program for rough machining and a first part program for finish machining must be created and prepared separately from each other. This is disadvantageous in that programming becomes complicated and a memory having a large capacity is required.